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Descriptions

Landforms sculpted by mass movements comprise much of the landscape in the Middle Santiam study area. Bedrock in the area is
mostly basalt and andesite flows and varied volcaniclastic rocks of the Little Butte Volcanic Series of Oligocene and early Miocene age, unconforniably overlain by andesite flows and tuffs of the Sardine
Formation of middle and late Miocene age. Some mass movements in the study area may have originally occurred during glacial or
interglacial periods of the late Pleistocene, although this is largely speculative. Active slump-earthflows, debris avalanches, and debris torrents impact streams, timber resources and man-made
structures. Earthflows are associated with intercalated lava flows and volcaniclastics, especially stratified volcaniclastics which have low strength, high plasticity, and contain montmorillonite, an expandable clay mineral. Debris avalanches are associated with non-cohesive soils on steep slopes.
Slump-earthflows show distinctive morphological and vegetative characteristics which reflect recency and rates of movement. Areas with different levels of activity can be mapped and the data used for certain land-planning applications. Surface movement rates can be measured on active earthflows by
conventional surveying and the use of stake arrays. Results indicate that rapid surface movement exceeding 20 ft/yr is occurring on at least two earthflows, and that intraannual and annual periods of accelerated movement coincide with periods of greater water input from precipitation and snowmelt Movement of the Jude Creek
earthflow also appears to be related to erosion of the toe by Jude Creek Movement on the Middle Santiam earthflow has greatly
accelerated in the last three years (since 1978), compared with average rates for the previous 13 years Road construction in 1965
preceded the most recent pulse of movement at this site Inventory of debris avalanches in the study area indicate a link between road construction, storm history and debris avalanche occurrence Rates of soil transfer for road-related events is much greater than those for either forested or clearcut events.